The present invention relates to a small-amplitude differential interface circuit useful as a fast interface or the like between semiconductor devices.
According to technique disclosed in U.S. Pat. No. 5,418,478, a driver circuit for a small-amplitude differential interface is composed of two sourcing current generators, two sinking current generators and four switching devices. Alternatively, according to technique disclosed in U.S. Pat. No. 6,111,431, a driver circuit for a small-amplitude differential interface is composed of one sourcing current generator, one sinking current generator and four switching devices. In either technique, signals are transferred by switching the direction of an output current with an H bridge.
In order to reduce the voltage of a voltage source, the two switching devices disposed on the side of the sourcing current generators in U.S. Pat. No. 5,418,478 may be omitted so as to set the current values of the two sinking current generators to be double the current value of each sourcing current generator. However, if the both switching devices are simultaneously in an on-state in switching the polarity of the output, the relationship that a current sourced by external load capacitance is equivalent to a current sunk by the external load capacitance is spoiled, and therefore, the charge amount of the external load capacitance is changed, resulting in varying a DC (continuous current) level. When the DC level is varied, a margin against ground level variation between a transmitter and a receiver is reduced, which makes communications impossible in the worst case. Such a simultaneous on-state occurs when the polarity of the output is changed, and hence, the influence appears more seriously as the frequency of the output signal is higher. Since rapid switching is demanded, it is difficult to overcome this problem.
In the architecture disclosed in U.S. Pat. No. 6,111,431, although the DC level is not varied, since two switching devices are inserted in a current path, the voltage is largely lowered by these switching devices, which causes a problem when the supply voltage is to be lowered.